Here's how you can get a word's definition by using a gesture in OS X on your Mac.
1. Use a single tap with three fingers on the word to see the definition.
2. Note that you must have the "Look Up - Tap with Three Fingers" gesture turned on in System Preferences>Trackpad.
For more, see the original article at the link below.
5 Essential Time-Saving Tips for OS X Mountain Lion | TechnoDuet